The manufacture by continuous production processes, such as extrusion molding, of insulated wire or cable and similar elongated products comprising polymeric materials which crosslink cure upon raising their temperature to a particular level, require ancillary means and a secondary operation to heat the continuously moving shaped polymeric material emanating from the molding device to its crosslinking curing condition substantially uniformly through the mass thereof.
A common means of imparting the temperatures necessary to effect a crosslink curing within a continuously moving polymeric material or product thereof, comprises conveying the molded-to-shape material or product through a body of a heating fluid such as steam in sequence with and at the same rate as its formation in the continuous molding operation. Conventional systems for heating fast moving products of high rate production operations with fluids such as steam require retention chambers or enclosures of extensive lengths. Steam curing chambers, for example, can extend for about 200 feet or more to provide sufficient dwell time therein for the fast moving products to achieve both the necessary heating and cooling uniformly through the body of the material under a pressure adequate to offset the deleterious expansion of any gases within the material. The space requirements as well as costs and cumbersome operating conditions, among other handicaps, for the aforesaid fluid type of heating systems have prompted the proposal of more compact heating means such as electrical or other heat generating devices employed in connection with the extruding unit or other continuous forming apparatus. For example, electrical heating units have been applied about or within extruder barrels, the area of their outlet or die, and the like portions of extruding apparatus.
However, heating systems or units associated with or applied to the extruding unit, or simply with an enclosed continuous forming apparatus, are especially troublesome when molding polymeric materials which crosslink to an infusible or permanently heat stable state upon the application of heat because of the possible occurrence of a premature crosslinking of the polymeric material while within the forming apparatus, molding die, or other enclosed portions of the system. A substantial degree of crosslinking of the polymeric material prior to its molding to shape and discharge from the confining die or shape imparting means not only disrupts the molding or degrades the product itself, it also can obstruct the die or extruder or other enclosure and thereby terminate production which requires a substantial effort to free the apparatus and resume the production system in addition to the loss of products.
Moreover, the problem of premature crosslinking of heat activated polymeric materials is very often accentuated by the fact that most crosslinking reactions are exothermic whereby once initiated, the crosslinking action progresses autogentically or independent of external heat sources. Also, the fast or responsive control or changing of thermal conditions within and about typical extruding apparatus is all but impossible because of the relatively high specific heat and heat sink effect of their massive steel or other metal bodies which enclose the heat activatable polymeric material during its plasticizing and molding.